Catálogo de publicaciones - libros

Resolving the atomic and electronic structures of nanoclusters represents an important preliminary for their controlled use in future nanotechnologies. Here we show through the comparison of density-functional calculations with high-resolution photoelectron spectroscopy that 1.4 nm nanoparticles of silver (negatively charged clusters of 53 to 58 atoms) are icosahedral-based structures displaying a perfect icosahedral-induced electronic shell structure for Ag and slightly perturbed shell structures for the neighboring cluster sizes. At variance, 55-atom gold clusters exhibit several isomeric structures of low symmetry, with a largely diminished electronic shell structure. This surprising qualitative difference is attributed to strong relativistic bonding effects in gold.

Large scale molecular dynamics (MD) computer simulations are used to study the amorphous alkali silicates (LiO)(2·SiO) [LS2], (KO)(2·SiO) [KS2], and (0.5·LiO)(0.5·KO)(2·SiO) [LKS2]. These systems are characterized by a fast alkali ion motion in a relatively immobile Si-0 matrix. We investigate the so-called mixed alkali effect (MAE) which is reflected as a significant decrease of the alkali ion diffusion constants in LKS2 as compared to the corresponding binary systems LS2 and KS2. We show that the subtle interplay between the structure on intermediate length scales and the alkali diffusion is important to understand the microscopic origin of the MAE.

The adsorption on methylchloride (CHCl) on the Si(001) surface is studied by calculations using the gradient-corrected density-functional theory (DFT-GGA) together with ultrasoft pseudopotentials and a plane-wave basis set. The energetically favoured adsorption geometries are examined with respect to their bandstructures, surface dipoles and charge transfer characteristics.

Quantum Monte Carlo simulations are used to study the dynamics and the critical properties of strongly correlated systems relevant to the fields of cold quantum gasses and high- superconductivity. Recent advances in cooling techniques of quantum gasses allow to reach the degenerate regime for fermionic samples. Loading these systems on optical lattices can bring the gas to a strongly correlated regime. We analyze the properties of trapped degenerate Fermi gasses on optical lattices and show that they display quantum critical behavior and universality at the boundaries between metallic and Mott insulating phases. On our other field of interest, high- superconductivity, a Quantum Monte Carlo algorithm we developed recently is used to study the dynamics of the nearest-neighbor (n.n) model relevant to the low energy properties of the copper oxides materials. We show that antiholons identified in the supersymmetric inverse squared (ISE) model are generic excitation of the n.n. model since they are clearly visible in the single-particle spectral function of the n.n. model in the whole Luttinger-liquid regime. We have further shown that even the analysis of the two-particle spectral functions of the n.n. model can be based on the elementary excitations of the ISE model.

We present a numerical study of the doping dependence of the spectral function of the n-type cuprates. Using cluster-perturbation theory and the self-energy-functional approach, we calculate the spectral function of the Hubbard model with next-nearest neighbor electronic hopping amplitude = −0.35 and on-site interaction = 8 at half filling and doping levels ranging from = 0.077 to = 0.20. We show that a comprehensive description of the single particle spectrum of the electron doped cuprates is only possible within a strongly correlated model. Weak coupling approaches that are based upon a collapse of the Mott gap by vanishing on-site interaction are ruled out.

Numerical simulation of complex flows has always demanded the biggest com-puters both in storage capacity and in performance that were available on the market. This situation is still going on. The following paragraph repre-sents a selection of papers that were submitted as yearly demanded progress reports to the HLRS. Although most of the reports revealed a very high sci-entific standard those papers were preferably selected for publication that clearly demostrated the unalterable usage of high performance computers (HFC) for the solution of the problem.

Investigations on laminar-turbulent transition for high-speed flows at hypersonic Mach-numbers will be presented. Dissociation takes place above a temperature of T>2000K within the boundary layer, a temperature which is reached easily at Mach-numbers above M=5. Additional degrees of freedom for the energy must be taken into account by employing a vibrational energy equation. Chemical reactions take place which are modeled by a 5-species model proposed by Park []. Further details on the chemical modeling can be found in [, ].

Controlled disturbances can be introduced by means of a disturbance strip at the wall which is also capable to model point source disturbances. Results will be shown for free-flight conditions at an altitude of H=50Km and at a speed of M=20. Experiments for qualitative validation of the results are available in [].

By means of spatial direct numerical simulations (DNS) based on the complete Navier-Stokes equations the effect of three-dimensional discrete suction on the spatial development of a laminar boundary-layer flow generic for the front part of a swept-back airliner wing has been investigated. The baseflow is an accelerated Falkner-Skan-Cooke boundary layer, on a swept wedge with semi-opening angle of 45° (Hartree parameter = 0.5) which is mainly characterised by crossflow instability. The simulations of the microscale phenomena confirm that 3-d suction at the wall can excite unstable crossflow disturbances that have to be minimised by using either slot arrays or hole arrays with high porosity, otherwise the stabilising (2-d) effect of suction is compromised. Premature transition through oversuction could be identified as a convective secondary instability of the flow field deformed by strong steady crossflow vortices emerging from the suction panel.

Shock-wave/turbulent-boundary-layer interaction compression-ramp flow is a canonical test configuration for statistical turbulence modeling. Extensive experimental data are available, whereas computational data focus mainly on Reynolds-averaged computations employing a wide range of turbulence models. In figure 1 basic flow features are sketched [Zhe91]. The undisturbed incoming turbulent boundary layer interacts with the shock wave, for suffi-ciently large deflection angles resulting in a separation region near the com-pression corner, and a A-shock system containing the separation region. Sub-sequently the disturbed boundary layer passes through the Prandtl-Meyer expansion near the decompression corner and finally relaxes towards a devel-oped zero-pressure-gradient boundary layer.

In this work the large-eddy simulation (LES) is used to investigate incompressible flow around a sphere with trip wire. The sphere is located in a channel with square cross-section, and the bulk Reynolds number is = 50 000. The computational effort implied by demands for sufficient spatial and temporal resolution of the flow structures requires parallel runs on a high-performance computer. The numerical results are compared to the experimental ones in order to provide reliable data for testing, calibrating and improvement of statistical turbulence models. The time-averaged LES-results and the measured data obtained by the laser-Doppler-anemometry (LDA) for the velocity and the Reynolds-stress components are in reasonable agreement. Accuracy of the predicted mean-flow velocity component is particularly good. Comparison of the Reynolds stresses shows certain deviations in the far wake, agreement is however acceptable from the qualitative point of view.